Re: [PATCH] c++: Don't defer local statics initialized with constant expressions [PR108702]

[Jason Merrill <jason@redhat.com>]

On 2/9/23 11:14, Jakub Jelinek wrote:
> Hi!
> 
> The stmtexpr19.C testcase used to be rejected as it has a static
> variable in statement expression in constexpr context, but as that
> static variable is initialized by constant expression, when P2647R1
> was implemented we agreed to make it valid.
> 
> Now, as reported, the testcase compiles fine, but doesn't actually link
> because the static variable isn't defined anywhere, and with -flto ICEs
> because of this problem.  This is because we never
> varpool_node::finalize_decl those vars, the constant expression in which
> the DECL_EXPR is present for the static VAR_DECL is folded (constant
> evaluated) into just the address of the VAR_DECL.

Would it make sense to define it when we see the DECL_EXPR in constant 
evaluation?

> Now, similar testcase included below (do we want to include it in the
> testsuite too?) works fine, because in
> cp_finish_decl -> make_rtl_for_nonlocal_decl
> we have since PR70353 fix:
>    /* We defer emission of local statics until the corresponding
>       DECL_EXPR is expanded.  But with constexpr its function might never
>       be expanded, so go ahead and tell cgraph about the variable now.  */
>    defer_p = ((DECL_FUNCTION_SCOPE_P (decl)
>                && !var_in_maybe_constexpr_fn (decl))
>               || DECL_VIRTUAL_P (decl));
> and so don't defer them in constexpr/consteval functions.  The following
> patch extends that and doesn't defer vars initialized by constant
> expressions either, because otherwise there is nothing to finalize those.
> It is true that e.g. with -O0
> int foo (int x) {
>    if (x) { static int y = 1; ++y; }
>    if (0) { static int z = 1; ++z; }
>    return sizeof (({ static int w = 1; w; }));
> }
> we used to emit just y and z and with the patch emit also w, but with
> optimizations that is optimized away properly.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> The testcase I was talking above that works because of the
> && !var_in_maybe_constexpr_fn (decl) case is:
> 
> extern "C" void abort ();
> 
> constexpr const int *
> foo ()
> {
>    static constexpr int a = 1;
>    return &a;
> }
> 
> consteval const int *
> bar ()
> {
>    static constexpr int a = 1;
>    return &a;
> }
> 
> [[gnu::noipa]] void
> baz (const int *x)
> {
>    if (*x != 1)
>      abort ();
> }
> 
> int
> main ()
> {
>    constexpr const int *p = foo ();
>    constexpr const int *q = bar ();
>    baz (p);
>    baz (q);
>    if (p == q)
>      abort ();
> }
> 
> 2023-02-09  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/108702
> 	* decl.cc (make_rtl_for_nonlocal_decl): Don't defer local statics
> 	initialized by constant expressions.
> 
> 	* g++.dg/ext/stmtexpr19.C: Use dg-do link rather than dg-do compile.
> 
> --- gcc/cp/decl.cc.jj	2023-01-24 11:10:13.151076134 +0100
> +++ gcc/cp/decl.cc	2023-02-09 13:29:50.527083618 +0100
> @@ -7731,9 +7731,12 @@ make_rtl_for_nonlocal_decl (tree decl, t
>   
>     /* We defer emission of local statics until the corresponding
>        DECL_EXPR is expanded.  But with constexpr its function might never
> -     be expanded, so go ahead and tell cgraph about the variable now.  */
> +     be expanded, so go ahead and tell cgraph about the variable now.
> +     Also don't defer local statics initialized by constant expressions,
> +     see PR108702.  */
>     defer_p = ((DECL_FUNCTION_SCOPE_P (decl)
> -	      && !var_in_maybe_constexpr_fn (decl))
> +	      && !var_in_maybe_constexpr_fn (decl)
> +	      && !DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (decl))
>   	     || DECL_VIRTUAL_P (decl));
>   
>     /* Defer template instantiations.  */
> --- gcc/testsuite/g++.dg/ext/stmtexpr19.C.jj	2022-11-19 09:26:30.168061316 +0100
> +++ gcc/testsuite/g++.dg/ext/stmtexpr19.C	2023-02-09 13:32:48.887453520 +0100
> @@ -1,6 +1,6 @@
>   // PR c++/81073
>   // { dg-options "" }
> -// { dg-do compile { target c++11 } }
> +// { dg-do link { target c++11 } }
>   
>   struct test { const int *addr; };
>   
> 
> 	Jakub
>